In recent years, flat displays are used in a number of fields and places, and advances in informatization has rendered them more and more important. At the present time, liquid crystal displays (hereinafter referred to as “LCDs”) are representative of flat displays. Organic electroluminescence (hereinafter referred to as “organic EL”), inorganic electroluminescence (hereinafter referred to as “inorganic EL”), plasma display panels (hereinafter referred to as “PDP”), light emitting diode display devices (hereinafter referred to as “LED”), vacuum fluorescent display devices (hereinafter referred to as “VFD”), field emission display (hereinafter referred to as “FED”) and the like are also under active development. All of these new flat displays are called “selfluminous type” and are greatly different from LCDs in the following points and have excellent features not possessed by LCDs. Among others, research in organic ELs is particularly actively made.
The organic ELs have advantages such as a high level of visibility by virtue of self-color development, excellent impact resistance because of a solid state display unlike a liquid crystal display, a high response speed, less susceptibility to a temperature change, and a high visible angle. In recent years, attention has been drawn to the utilization of organic ELs as a light emitting element in image display devices.
Regarding the method for EL layer formation, in general, when a low-molecular weight material is used as the material for the EL layer, a vacuum vapor deposition method using a mask is used, while, when a polymeric material is used, a solution is prepared from the material and is used for ink jet recording, spin coating, printing, transfer or the like to form the EL layer.
In recent years, coatable low-molecular materials are also reported. In the vacuum deposition of the low-molecular weight material using a mask among them, an increase in size of vacuum devices and vapor deposition masks is difficult. This makes it difficult to cope with an increase in size and to prepare a large number of organic EL elements using a large substrate. The above difficulty poses no problem in experimental preparation in a development stage. In a full-scale production stage, however, this is disadvantageous in market competitiveness from the viewpoints of tact and cost. On the other hand, for polymeric materials or coatable low-molecular weight materials, film formation by wet coating methods such as ink jet recording, printing, casting, alternate adsorption, spin coating, and dipping is possible and thus can cope with large substrates, and these are promising methods for organic EL element formation.
In the above flat displays including LCDs, a device element is usually formed on a glass substrate. The glass substrate is used because of its excellent transparency, optical isotropy, gas barrier properties, chemical resistance, smoothness, dimension stability and the like. The glass substrate, however, lacks in flexibility. Therefore, when the glass substrate is used, for example, in displays for small notebook-sized personal computers and portable terminals such as electronic notepads, the glass is sometimes damaged.
On the other hand, plastics have features such as flexibility, light weight, and possible long sheet processing in a roll form and have drawn attention as a substrate material for displays. When the plastic sheet is used solely, however, properties required of the above substrate material cannot be satisfied.
To overcome the above problem, in recent years, a composite structure comprising a flexible thin glass and a plastic has been proposed. For example, Japanese Patent Laid-Open No. 329715/1999 discloses a flexible substrate for an organic device which is a not more than 200 μm-thick composite structure comprising a thin glass layer and a plastic layer. Published Japanese Translation of PCT Publication No. 534305/2002 discloses a glass/plastic composite film comprising a 1 to 200 μm-thick polymer layer provided on at least one surface of a 10 to 500 μm-thick glass film. Further, Japanese Patent Laid-Open No. 50565/2004 discloses a thin sheet substrate comprising an inorganic glass layer and a resin layer formed of a composition composed mainly of a metal oxide polymer. The claimed advantage of the thin sheet substrate disclosed in Japanese Patent Laid-Open No. 50565/2004 is to have a high heat resistance temperature, i.e., substantially no thermogravimetric reduction up to a temperature around 400° C., gas barrier properties on a level comparable with the glass, a high level of transparency to visible light, and a high level of flexibility of 6 mm in terms of limit curvature radius.
The flexible substrate disclosed in Japanese Patent Laid-Open No. 329715/1999, however, disadvantageously has low heat resistance. The composite film disclosed in Published Japanese Translation of PCT Publication No. 534305/2002 has high heat resistance. However, the heat resistant temperature is not yet satisfactory and is about 280° C. Therefore, the step of preparing a device element on the composite film is disadvantageously limited to 280° C. or below, and the low heat resistance of the film poses a problem, for example, in the film formation of a transparent electrode of indium tin oxide (ITO) or the like. The resin layer formed of the composition composed mainly of the metal oxide polymer constituting the thin sheet substrate disclosed in Japanese Patent Laid-Open No. 50565/2004 is disadvantageously easily swollen or dissolved in an organic solvent, particularly an aromatic organic solvent. Specifically, in the production of an organic device, for example, when an organic EL element is produced using the substrate disclosed in Japanese Patent Laid-Open No. 50565/2004, a transparent electrode of ITO or the like is formed in a predetermined pattern as an anode on a resin layer formed of a composition composed mainly of a metal oxide polymer stacked onto a glass layer. In this case, the resin layer as the underlying layer is exposed from between the patterned transparent electrode. When an organic EL layer is formed by a wet coating process such as a spin coating method, the resin layer as the underlying layer exposed from between the transparent electrode pattern is disadvantageously swollen or dissolved in the aromatic organic solvent dissolving the organic EL layer. Therefore, the thin sheet substrate disclosed in Japanese Patent Laid-Open No. 50565/2004 cannot be used in a production process using a solvent such as an aromatic organic solvent, and, in polymeric organic EL materials, which are dissolved in a solvent such as an aromatic organic solvent to prepare a solution for coating, and coatable low-molecular weight organic EL materials, disadvantageously, film formation by highly productive and practical wet coating methods, such as ink jet recording, printing, spin coating, and dipping, cannot be carried out.